1. Field of the Invention
The present invention relates to detectable indicators, including fluorescent indicators, having increased resistance to oxidation.
2. Description of the Related Art
Fluorescent molecules are used for a range of applications including fabric and color brighteners, signs, various inks for printing, diagnostics as tags and probes when linked to antibodies or other molecules, and can be configured at a molecular level to be used as chemical and biochemical active indicators specifically designed to detect certain analytes, for example glucose.
The complexation of carbohydrates, including glucose, with phenylboronic acid has been known for a long time and the reversibility of that interaction has served as a basis for the chromatographic separation of sugars. Specifically, in 1959, Lorand and Edwards reported association constants for aqueous associations of phenylboronic acid with many saturated polyols; binding interactions ranged from very weak (e.g., ethylene glycol, Kd=360 mM) to moderately strong (e.g., glucose, Kd=9.1 mM). See J. Yoon, et al., Bioorganic and Medicinal Chemistry 1(4):267-71 (1993). The binding mechanism is believed to occur through displacement of the hydroxyl groups on a boronate moiety with adjacent hydroxyl groups on glucose.
U.S. Pat. No. 5,503,770 (James, et al.) describes a fluorescent boronic acid-containing compound that emits fluorescence of a high intensity upon binding to saccharides, including glucose. The fluorescent compound has a molecular structure comprising a fluorophore, at least one phenylboronic acid moiety and at least one amine-providing nitrogen atom where the nitrogen atom is disposed in the vicinity of the phenylboronic acid moiety so as to interact intramolecularly with the boronic acid. Such interaction thereby causes the compound to emit fluorescence upon saccharide binding. See also T. James, et al., J. Am. Chem. Soc. 117(35):8982-87 (1995).
Additionally, fluorescent sensors using an anthrylboronic acid-containing compound for detecting blood glucose are known in the art. For example, J. Yoon, et al., J. Am. Chem. Soc. 114:5874-5875 (1992) describe that anthrylboronic acid can be used as a fluorescent chemosensor for signaling carbohydrate binding, including binding of glucose and fructose.
Fluorescent molecules are susceptible to degradation, where they lose fluorescence intensity (or brightness) over time by often variable rates of oxidation. The oxidation may be commonly associated with photobleaching, which is technically “photo-oxidation”, or may be oxidized by various reactive oxygen species within the local environment of the fluorescent molecule. Any number of potential oxidants exist in the environment and atmosphere such as ozone, or may exist inside a living body ranging from humans to bacteria. Inside a living body, normal reactive oxygen species (ROS) can include those involved in typical healthy healing reactions such as peroxide, hydroxyl radicals, peroxynitrite, superoxide, and others. Inside a living system there are also specific enzymes called oxygenases for the specific purpose of oxidation in the breakdown of molecules. An adverse result of reactive oxygen species or oxygenase activity on a fluorescent molecule is typically loss of fluorescence. In the case of an indicator molecule, or a passive tag, probe, or label, the useful life and sensitivity of the device, or diagnostic, is limited, or may be rendered completely ineffective by oxidative degradation of fluorescent signal. Therefore, there remains a need for fluorescent molecules that have increased resistance to oxidation.